Ion implantation to form trench-bottom oxide of MOSFET

ABSTRACT

Disclosed herein are methods for forming MOSFETs. In some embodiments, a method may include providing a device structure including a plurality of trenches, and forming a mask over the device structure including within each of the plurality of trenches and over a top surface of the device structure. The method may further include removing the mask from within the trenches, wherein the mask remains along the top surface of the device structure, and implanting the device structure to form a treated layer along a bottom of the trenches. In some embodiments, the method may further include forming a gate oxide layer along a sidewall of each of the trenches and along the bottom of the trenches, wherein a thickness of the oxide along the bottom of the trenches is greater than a thickness of the oxide along the sidewall of each of the trenches.

FIELD OF THE DISCLOSURE

The present embodiments relate to semiconductor device patterning, and more particularly, to MOSFETs with increased trench-bottom oxide thickness.

BACKGROUND OF THE DISCLOSURE

Low voltage power metal-oxide-semiconductor field-effect transistors (MOSFETs) are often used in load switching applications where reduction of the on-resistance (R_(ds)) of the device is desirable. In some applications, the R_(ds)A of the device is minimized, where R_(ds)A is the on-resistance of the device multiplied by the active area of the device. Additionally, low voltage power MOSFETs are commonly used in high frequency DC-DC applications.

Trench-based MOSFETs are often used to reduce R_(ds)A of the device. However, conventional thermal oxidation processes lead to non-uniform oxide in the trench. In some cases, a reduction in trench width causes trench-bottom oxide thinning due to oxygen diffusion or due to crystal orientation (e.g., for SiC trenches), resulting in degraded breakdown voltage. Furthermore, trench MOSFET scaling also requires a thicker bottom oxide to reduce gate charge (Qg). Accordingly, improved trench oxide formation approaches are needed to maximize scalability.

SUMMARY

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended as an aid in determining the scope of the claimed subject matter.

In one aspect, a method may include providing a device structure including a plurality of trenches, forming a mask over the device structure, including within each of the plurality of trenches and over a top surface of the device structure, removing the mask from within each of the plurality of trenches, wherein the mask remains along the top surface of the device structure, and implanting the device structure to form a treated layer along a bottom of each of the plurality of trenches.

In another aspect, a method of forming a MOSFET may include providing a device structure including a well formed in an epitaxial layer, forming a plurality of trenches through the well and the epitaxial layer, and forming a mask over the device structure including within each of the plurality of trenches and over a top surface of the device structure. The method may further include removing the mask from within each of the plurality of trenches, wherein the mask remains along the top surface of the device structure, and implanting the device structure to form a treated layer along a bottom of each of the plurality of trenches.

In yet another aspect, a method of forming a MOSFET trench-bottom oxide may include providing a device structure including a well formed in an epitaxial layer, forming a plurality of trenches through the well and the epitaxial layer, forming a hardmask over the device structure including within each of the plurality of trenches and over a top surface of the device structure, and removing the hardmask from within each of the plurality of trenches, wherein the hardmask remains along the top surface of the device structure. The method may further include implanting the device structure to form a treated layer along a bottom of each of the plurality of trenches, and forming a gate oxide layer along a sidewall of each of the plurality of trenches and along the bottom of each of the plurality of trenches, wherein a thickness of the gate oxide layer along the bottom of each of the plurality of trenches is greater than a thickness of the gate oxide layer along the sidewall of each of the plurality of trenches.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate exemplary approaches of the disclosure, including the practical application of the principles thereof, as follows:

FIG. 1 is a cross-sectional side view of a device structure, according to embodiments of the present disclosure;

FIG. 2 is a side cross-sectional view illustrating the device following formation of a hardmask, according to embodiments of the present disclosure;

FIG. 3 is a side cross-sectional view illustrating the device following an etch process, according to embodiments of the present disclosure;

FIG. 4 is a side cross-sectional view illustrating an implantation to the device structure, according to embodiments of the present disclosure;

FIG. 5 is a side cross-sectional view illustrating formation of a gate oxide layer over the device structure, according to embodiments of the present disclosure;

FIG. 6 is a side cross-sectional view illustrating formation of a gate material, according to embodiments of the present disclosure; and

FIG. 7 illustrates a schematic diagram of a processing apparatus according to embodiments of the present disclosure.

The drawings are not necessarily to scale. The drawings are merely representations, not intended to portray specific parameters of the disclosure. The drawings are intended to depict exemplary embodiments of the disclosure, and therefore are not be considered as limiting in scope. In the drawings, like numbering represents like elements.

Furthermore, certain elements in some of the figures may be omitted, or illustrated not-to-scale, for illustrative clarity. The cross-sectional views may be in the form of “slices”, or “near-sighted” cross-sectional views, omitting certain background lines otherwise visible in a “true” cross-sectional view, for illustrative clarity. Furthermore, for clarity, some reference numbers may be omitted in certain drawings.

DETAILED DESCRIPTION

Methods and devices in accordance with the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, where various embodiments are shown. The methods and devices may be embodied in many different forms and are not to be construed as being limited to the embodiments set forth herein. Instead, these embodiments are provided so the disclosure will be thorough and complete, and will fully convey the scope of the methods to those skilled in the art.

Embodiments described herein advantageously provide a bottom-trench oxide formation process for MOSFET devices. In some embodiments, an ion implantation is performed to a bottom surface of the device trenches to increase oxide growth during a subsequent thermal oxide process. The ion implantation is capable of enabling a flexible, thick bottom oxide to maintain a constant cell structure and maximize scalability.

FIG. 1 is a side cross-sectional view of a semiconductor device structure (hereinafter “device”) 100, such as a MOSFET, according to one or more embodiments described herein. The device 100 may include a device structure 101 having a substrate 102, an epitaxial layer 104, a well 106, and a source region or layer 108. Although non-limiting, the epitaxial layer 104 may be a silicon carbide (SiC) n-type drift layer, the well 106 may be a p-type well, and the source layer 108 may be an N⁺ source layer. In other embodiments, the source layer 108 may include N+ and P+ regions next to each other, wherein P+ serves as a PWell contact and N+ serves as the source layer. Shown as a single layer, the epitaxial layer 104 may include multiple layers in other embodiments. As known, the well 106 may be formed using a plurality of doping steps or epitaxy steps.

As further shown, a plurality of trenches 112 may be formed in the device 100, e.g., using one or more blocking and vertical etch processes to reach the sidewall slope from 80 to 90 degrees. The trenches 112 may be defined, in part, by a sidewall 114 and a bottom surface 116. As demonstrated, the trenches 112 may be formed after the well 106 and source layer 108 are formed.

As shown in FIG. 2 , a mask (e.g., hardmask) 110 may then be formed over the device structure 101, including within each of the trenches 112. In some embodiments, the mask 110 may be a non-uniform layer of silicon nitride (SiN) or other mask layer(s) formed using a conformal deposition process, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), low-pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), or atomic layer deposition (ALD). As shown, the mask 110 may be formed along the sidewall 114 and the bottom surface 116 of each trench 112. The mask 110 may also be formed along a top surface 118 of the device structure 101. In exemplary embodiments, the mask 110 atop the device structure 101 has a first thickness ‘T1’ (e.g., in the y-direction), while the mask 110 along the sidewall 114 has a second thickness ‘T2’ (e.g., in the x-direction) and the mask 110 along the bottom surface 116 has a third thickness ‘T3’ (e.g., in the y-direction), wherein T1>T2 and T3.

In FIG. 3 , the mask 110 may be partially removed from the device 100 using, e.g., an etch process 122. As shown, the mask 110 may be removed from the sidewall 114 and the bottom surface 116 of each trench 112. Meanwhile, the mask 110 remains along the top surface 118 of the device structure 101 due to the different mask thicknesses (e.g., T1, T2, T3) along the top surface 118 versus along the surfaces of the trenches 112. As such, the source layer 108 advantageously remains protected. In various embodiments, the etch process 122 may include a vertical and/or angled ion etch, or an isotropic wet etch. For example, the etch process 122 may include one or more vertical etch cycles in which the mask 110 is removed from the bottom surface 116 of each trench 112. One or more angled ion etch cycles may then be performed to remove the mask 110 from the sidewall 114 of each trench 112. In the case of an isotropic wet etch, because T1 is thicker than both T2 and T3, the mask 110 is removed from the sidewall 114 and the bottom surface 116 of the trenches 112 without being completely removed from along the top surface 118.

FIG. 4 demonstrates an ion implant 130 performed to the device 100 to form a treated layer 132 along the bottom surface 116 of each trench 112. In some embodiments, the treated layer 132 is an amorphized area/layer or implanted layer of the epitaxial layer 104. As shown, the treated layer 132 is not formed along the sidewall 114 of the trenches 112. In some embodiments, the ion implant 130 may be a beamline implant delivered at a pre-defined angle, a predefined energy, a predefined dose, etc. The bottom surface 116 may be implanted until a desired dopant concentration is achieved. In some embodiments, multiple species can be used to increase oxidation rate of a subsequently grown trench oxide layer. Although non-limiting, the implant angle may be perpendicular, or approximately perpendicular, to a plane defined by the top surface 118 of the device structure 101 to implant a pre-defined area and depth. For example, the ion implant 130 may impart ions at approximately 0° incidence angle to the sidewall 114 (or approximately 90° relative to the top surface 118 of the device structure 101) to influence subsequent formation of the trench oxide layer within the trenches 112. The implantation angle may vary in other embodiments, e.g., by +/−15°. By selecting an appropriate angle of delivery, the ion implant 130 may impact just the bottom surface 116 of each trench 112 and the mask 110 without impacting the sidewall 114, especially the portion of the sidewall 114 defined by the well 106 and the source layer 108.

In some embodiments, the ion implant 130 may be a cryogenic ion implant. For example, one or more ion implants may be performed at a low temperature, e.g., −100 degrees Celsius (C) to advantageously form a deeper amorphous layer having lower energy requirements and thus easier design requirements for the mask 110. Furthermore, the cryogenic ion implant results in lower end-of-range (EOR) defect density and easier oxidation control. In some embodiments, the cryogenic ion implant includes chilling a platen (not shown) upon which the device structure 101 is provided. In yet other embodiments, the ion implant may be a room temperature (RT) ion implant.

FIG. 5 demonstrates formation of a gate oxide layer 140 over the device 100, including within the trenches 112, after the mask 110 is removed. In other embodiments, the gate oxide layer 140 may be formed within the trenches 112 while the mask 110 is present. As shown, the gate oxide layer 140 may also be formed along the top surface 118 of the device structure 101. More specifically, the gate oxide layer 140 is formed by a thermal oxidation process to device 100, including to the treated layer 132 and the sidewall 114 of the trenches 112. In exemplary embodiments, the gate oxide layer 140 at the bottom of each trench 112 has a first thickness ‘T1’ (e.g., in the y-direction), while the gate oxide layer 140 along the sidewall 114 and along the top surface 118 has a second thickness ‘T2’, wherein T1>T2. By increasing oxide thickness along the bottom of the trenches 112, critical electrical field and gate charges may advantageously be reduced, while breakdown and off-power may be improved.

As demonstrated in FIG. 6 , the trenches 112 may then be filled with a gate material 150. In some embodiments, the gate material 150 may be a polysilicon gate refill deposited over the device 100 and then planarized (e.g., via CMP) or etched back selective to the top surface 118 of the device structure 101. Although non-limiting, in various embodiments, the gate material 150 may be a p-type or n-type polysilicon, which is doped.

In some embodiments, a second gate material (not shown) may be formed over the gate material 150 to form a split gate, wherein the gate material is separated from the second gate material by an isolation layer. Although non-limiting, the gate material and the second gate material may be different materials, wherein the first gate material may be a p-type polysilicon, and the second gate material may be an active n-type polysilicon.

In sum, embodiments herein disclose a device and associated process flow for forming a MOSFET. By doping trench bottoms prior to gate material deposition, gate oxidation is increased, which improves overall MOSFET device scaling.

FIG. 7 illustrates a schematic diagram of a processing apparatus 200 useful to perform processes described herein. One example of a beam-line ion implantation processing apparatus is the Varian VIISTA® Trident, available from Applied Materials Inc., Santa Clara, Calif. The processing apparatus 200 may include an ion source 201 for generating ions. For example, the ion source 201 may provide an ion implant, such as the ion implant 130 demonstrated in FIG. 4 for forming the treated layers 132 in the device 100. The ion source 201 may also provide an ion etch, such as the etch process 122 of FIG. 3 . The processing apparatus 200 may also include a series of beam-line components. Examples of beam-line components may include extraction electrodes 203, a magnetic mass analyzer 211, a plurality of lenses 213, and a beam parallelizer 217. The processing apparatus 200 may also include a platen 219 for supporting a substrate 202 to be processed. The substrate 202 may be the same as the substrate 102 described above. The substrate 202 may be moved in one or more dimensions (e.g. translate, rotate, tilt, etc.) by a component sometimes referred to as a “roplat” (not shown). It is also contemplated that the processing apparatus 200 may be configured to perform heated implantation processes to provide for improved control of implantation characteristics, such as the ion trajectory and implantation energy utilized to dope the substrate.

In operation, ions of the desired species, for example, dopant ions, are generated and extracted from the ion source 201. Thereafter, the extracted ions 235 travel in a beam-like state along the beam-line components and may be implanted in the substrate 202. Similar to a series of optical lenses that manipulate a light beam, the beam-line components manipulate the extracted ions 235 along the ion beam. In such a manner, the extracted ions 235 are manipulated by the beam-line components while the extracted ions 235 are directed toward the substrate 202. It is contemplated that the apparatus 200 may provide for improved mass selection to implant desired ions while reducing the probability of undesirable ions (impurities) being implanted in the substrate 202.

In some embodiments, the processing apparatus 200 can be controlled by a processor-based system controller such as controller 230. For example, the controller 230 may be configured to control beam-line components and processing parameters associated with beam-line ion implantation processes. The controller 230 may include a programmable central processing unit (CPU) 232 that is operable with a memory 234 and a mass storage device, an input control unit, and a display unit (not shown), such as power supplies, clocks, cache, input/output (I/O) circuits, and the like, coupled to the various components of the processing apparatus 200 to facilitate control of the substrate processing. The controller 230 also includes hardware for monitoring substrate processing through sensors in the processing apparatus 200, including sensors monitoring the substrate position and sensors configured to receive feedback from and control a heating apparatus coupled to the processing apparatus 200. Other sensors that measure system parameters such as substrate temperature and the like, may also provide information to the controller 230.

To facilitate control of the processing apparatus 200 described above, the CPU 232 may be one of any form of general-purpose computer processor that can be used in an industrial setting, such as a programmable logic controller (PLC), for controlling various chambers and sub-processors. The memory 234 is coupled to the CPU 232 and the memory 234 is non-transitory and may be one or more of readily available memory such as random access memory (RAM), read only memory (ROM), floppy disk drive, hard disk, or any other form of digital storage, local or remote. Support circuits 236 may be coupled to the CPU 232 for supporting the processor in a conventional manner. Implantation and other processes are generally stored in the memory 234, typically as a software routine. The software routine may also be stored and/or executed by a second CPU (not shown) that is remotely located from the hardware being controlled by the CPU 232.

The memory 234 is in the form of computer-readable storage media that contains instructions, that when executed by the CPU 232, facilitates the operation of the apparatus 200. The instructions in the memory 234 are in the form of a program product such as a program that implements the method of the present disclosure. The program code may conform to any one of a number of different programming languages. In one example, the disclosure may be implemented as a program product stored on computer-readable storage media for use with a computer system. The program(s) of the program product define functions of the embodiments (including the methods described herein). Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, flash memory, ROM chips or any type of solid-state non-volatile semiconductor memory) on which information is permanently stored; and (ii) writable storage media (e.g., floppy disks within a diskette drive or hard-disk drive or any type of solid-state random-access semiconductor memory) on which alterable information is stored. Such computer-readable storage media, when carrying computer-readable instructions that direct the functions of the methods described herein, are embodiments of the present disclosure.

It is to be understood that the various layers, structures, and regions shown in the accompanying drawings are schematic illustrations. For ease of explanation, one or more layers, structures, and regions of a type commonly used to form semiconductor devices or structures may not be explicitly shown in a given drawing. This does not imply that any layers, structures, and/or regions not explicitly shown are omitted from the actual semiconductor structures.

In various embodiments, design tools can be provided and configured to create the datasets used to pattern the semiconductor layers of the device 100, e.g., as described herein. For example, data sets can be created to generate photomasks used during lithography operations to pattern the layers for structures as described herein. Such design tools can include a collection of one or more modules and can also be comprised of hardware, software or a combination thereof. Thus, for example, a tool can be a collection of one or more software modules, hardware modules, software/hardware modules or any combination or permutation thereof. As another example, a tool can be a computing device or other appliance running software, or implemented in hardware.

As used herein, a module might be implemented utilizing any form of hardware, software, or a combination thereof. For example, one or more processors, controllers, ASICs, PLAs, logical components, software routines or other mechanisms might be implemented to make up a module. In implementation, the various modules described herein might be implemented as discrete modules or the functions and features described can be shared in part or in total among one or more modules. In other words, as would be apparent to one of ordinary skill in the art after reading the Detailed Description, the various features and functionality described herein may be implemented in any given application and can be implemented in one or more separate or shared modules in various combinations and permutations. Although various features or elements of functionality may be individually described or claimed as separate modules, one of ordinary skill in the art will understand these features and functionality can be shared among one or more common software and hardware elements.

For the sake of convenience and clarity, terms such as “top,” “bottom,” “upper,” “lower,” “vertical,” “horizontal,” “lateral,” and “longitudinal” will be understood as describing the relative placement and orientation of components and their constituent parts as appearing in the figures. The terminology will include the words specifically mentioned, derivatives thereof, and words of similar import.

As used herein, an element or operation recited in the singular and proceeded with the word “a” or “an” is to be understood as including plural elements or operations, until such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present disclosure are not intended as limiting. Additional embodiments may also incorporating the recited features.

Furthermore, the terms “substantial” or “substantially,” as well as the terms “approximate” or “approximately,” can be used interchangeably in some embodiments, and can be described using any relative measures acceptable by one of ordinary skill in the art. For example, these terms can serve as a comparison to a reference parameter, to indicate a deviation capable of providing the intended function. Although non-limiting, the deviation from the reference parameter can be, for example, in an amount of less than 1%, less than 3%, less than 5%, less than 10%, less than 15%, less than 20%, and so on.

Still furthermore, one of ordinary skill will understand when an element such as a layer, region, or substrate is referred to as being formed on, deposited on, or disposed “on,” “over” or “atop” another element, the element can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on,” “directly over” or “directly atop” another element, no intervening elements are present.

As used herein, “depositing” and/or “deposited” may include any now known or later developed techniques appropriate for the material to be deposited including yet not limited to, for example: chemical vapor deposition (CVD), low-pressure CVD (LPCVD), and plasma-enhanced CVD (PECVD). Additional techniques may include semi-atmosphere CVD (SACVD) and high density plasma CVD (HDPCVD), rapid thermal CVD (RTCVD), ultra-high vacuum CVD (UHVCVD), limited reaction processing CVD (LRPCVD), metal-organic CVD (MOCVD), and sputtering deposition. Additional techniques may include ion beam deposition, electron beam deposition, laser assisted deposition, thermal oxidation, thermal nitridation, spin-on methods, physical vapor deposition (PVD), atomic layer deposition (ALD), chemical oxidation, molecular beam epitaxy (MBE), plating, evaporation.

While certain embodiments of the disclosure have been described herein, the disclosure is not limited thereto, as the disclosure is as broad in scope as the art will allow and the specification may be read likewise. Therefore, the above description is not to be construed as limiting. Instead, the above description is merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto. 

What is claimed is:
 1. A method, comprising: providing a device structure including a plurality of trenches; forming a mask over the device structure including within each of the plurality of trenches and over a top surface of the device structure, wherein a thickness of the mask along the top surface of the device structure is greater than a thickness of the mask within each of the plurality of trenches; removing the mask from within each of the plurality of trenches, wherein the mask remains along the top surface of the device structure; and implanting the device structure to form a treated layer along a bottom of each of the plurality of trenches.
 2. The method of claim 1, further comprising forming a gate oxide layer along a sidewall of each of the plurality of trenches and along the bottom of each of the plurality of trenches, wherein a thickness of the gate oxide layer along the bottom of each of the plurality of trenches is greater than a thickness of the gate oxide layer along the sidewall of each of the plurality of trenches.
 3. The method of claim 2, further comprising thermally oxidizing the device structure to form the gate oxide layer from the treated layer along the bottom of each of the plurality of trenches.
 4. The method of claim 2, further comprising forming a gate material over the gate oxide layer within each of the plurality of trenches.
 5. The method of claim 1, wherein forming the treated layer comprises one of: amorphizing a bottom surface of each of the plurality of trenches, and implanting the bottom surface of each of the plurality of trenches to a desired dopant concentration.
 6. The method of claim 1, wherein implanting the device structure to form the treated layer comprises performing a cryogenic ion implant or a room temperature ion implant.
 7. The method of claim 1, wherein providing the device structure comprises: providing an epitaxial layer over a substrate; providing a well within the epitaxial layer; and providing a source region over the well, wherein the plurality of trenches are formed through the epitaxial layer, the well, and the source region.
 8. The method of claim 1, wherein removing the mask comprises performing a vertical ion etch to the mask within the plurality of trenches.
 9. The method of claim 1, further comprising removing the mask from the top surface of the device structure after the treated layer is formed.
 10. A method of forming a MOSFET, comprising: providing a device structure including a well formed in an epitaxial layer; forming a plurality of trenches through the well and the epitaxial layer; forming a mask over the device structure including within each of the plurality of trenches and over a top surface of the device structure, wherein a thickness of the mask along the top surface of the device structure is greater than a thickness of the mask within each of the plurality of trenches; removing the mask from within each of the plurality of trenches, wherein the mask remains along the top surface of the device structure; and implanting the device structure to form a treated layer along a bottom of each of the plurality of trenches.
 11. The method of claim 10, further comprising forming a gate oxide layer along a sidewall of each of the plurality of trenches and along the bottom of each of the plurality of trenches, wherein a thickness of the gate oxide layer along the bottom of each of the plurality of trenches is greater than a thickness of the gate oxide layer along the sidewall of each of the plurality of trenches.
 12. The method of claim 11, further comprising thermally oxidizing the device structure to form the gate oxide layer from the treated layer along the bottom surface of each of the plurality of trenches.
 13. The method of claim 11, further comprising forming a gate material over the gate oxide layer within each of the plurality of trenches.
 14. The method of claim 10, wherein forming the treated layer comprises one of: amorphizing a bottom surface of each of the plurality of trenches, and implanting the bottom surface of each of the plurality of trenches to a desired dopant concentration.
 15. The method of claim 10, wherein implanting the device structure to form the treated layer comprises performing a cryogenic ion implant or a room temperature ion implant.
 16. The method of claim 10, wherein providing the device structure further comprises providing a source region over the well, wherein the plurality of trenches are formed through the source region.
 17. The method of claim 10, further comprising removing the mask from the top surface of the device structure after the treated layer is formed.
 18. A method of forming a MOSFET trench-bottom oxide, comprising: providing a device structure including a well formed in an epitaxial layer; forming a plurality of trenches through the well and the epitaxial layer; forming a hardmask over the device structure including within each of the plurality of trenches and over a top surface of the device structure, wherein a thickness of the hardmask along the top surface of the device structure is greater than a thickness of the hardmask within each of the plurality of trenches; removing the hardmask from within each of the plurality of trenches, wherein the hardmask remains along the top surface of the device structure; implanting the device structure to form a treated layer along a bottom of each of the plurality of trenches; and forming a gate oxide layer along a sidewall of each of the plurality of trenches and along the bottom of each of the plurality of trenches, wherein a thickness of the gate oxide layer along the bottom of each of the plurality of trenches is greater than a thickness of the gate oxide layer along the sidewall of each of the plurality of trenches.
 19. The method of claim 18, further comprising removing the hardmask from the top surface of the device structure prior to forming the gate oxide layer.
 20. The method of claim 18, further comprising thermally oxidizing the device structure to form the gate oxide layer from the treated layer along the bottom of each of the plurality of trenches. 